1. Related Field
The present invention relates to an optical device and, more particularly, to an optical deflector that can rapidly change the direction of a light beam based on the electrooptical effect.
2. Description of the Related Art
Two of the major aspects that can characterize our information age are Internet and virtual reality. In order to meet the demand of exponential growth in data bandwidth, Internet development has been moving rapidly toward the direction of all-optical networking. The bottleneck to the realization of true all-optic networking is mainly the lack of an optical deflector or switch that can quickly route optical signals to different optical fibers or channels. Meanwhile, the advancement in virtual reality depends on the capability of high fidelity image display. One potential approach to achieving high fidelity image display is to form the image directly on the retina, which, too, depends on the breakthrough of optical deflector technology, so that an image-modulated light beam can scan the retina at high speed with high resolution.
In the conventional art, two main types of optical deflectors are in use, i.e., electromechanical one and electrooptical one. The electromechanical type of optical deflectors deflects a light beam through a moving mirror. Although high deflecting angle and high resolution can be easily obtained with this type of optical deflectors, the low deflecting speed inherent to its mechanic movement prohibits its use in high speed optical routing or scanning. Even with the latest on-substrate-machined micro-electromechanical mirror (MEMM), the best switching time of this type is still as high as on the order of milliseconds. On the other hand, the electrooptical type of optical deflectors made with certain electrooptical materials is capable to deflect a light beam as fast as in the order of nanoseconds to picoseconds, as the deflection of light in this type of optical deflectors is caused by the change in refractive index, and no mechanical moving part is involved. Among various electrooptical deflector structures in the conventional art, waveguide-type optical deflectors have the advantage to be readily fabricated and integrated with other optical elements in a common substrate. However, due to the limited electrooptical effect, the typical xcex94n, the change in refractive index, is only in the order of 10xe2x88x923. As a result, the achievable deflecting angle is very small with the conventional electrooptical light deflectors.
The conventional waveguide-type optical deflector typically comprises a light guiding layer made of an electrooptical material sandwiched between a top cladding layer and a bottom cladding layer. When electrical fields with different polarities are generated between a bottom electrode and triangle-shaped top electrodes, a gradient of xcex94n is induced in the light guiding layer along a direction transverse to the light traveling direction in the optical waveguide. It causes the effective length of optical paths to be changed from edge to edge, resulting in a tilt of the optical wavefront, and, therefore a deflection of the light beam. The peak-to-peak deflection angle is given approximately by:                               Δ          ⁢                      xe2x80x83                    ⁢          θ                =                  4          ⁢                      xe2x80x83                    ⁢          Δ          ⁢                      xe2x80x83                    ⁢          n          ⁢                      xe2x80x83                    ⁢                                    L              D                        ·                          1                              n                0                                                                        (        1        )            
wherein, xcex94n is the induced change of refractive index in the light guiding layer under a driving voltage, L and D are respectively the length and width of optical waveguide, and, no is the refractive index in the observing medium. Although the deflection angle is proportional to the ratio of LID as shown in Eq. (1), this ratio can only be increased to a point at which the front of a presumedly focused light beam has been bent so much inside the waveguide that it starts to hit one of the two edges of the waveguide. Accordingly, the geometrically allowable maximum ratio of L/D is approximately as:                                           (                          L              D                        )                    max                =                              (                          n                              4                ⁢                                  xe2x80x83                                ⁢                Δ                ⁢                                  xe2x80x83                                ⁢                n                                      )                                1            2                                              (        2        )            
Consequently, the achievable maximum peak-to-peak deflection angle with the conventional waveguide optical deflectors is:                                           (                          Δ              ⁢                              xe2x80x83                            ⁢              θ                        )                    max                =                              2            ⁢                          xe2x80x83                        ⁢                                          (                                  Δ                  ⁢                                      xe2x80x83                                    ⁢                                      n                    ·                    n                                                  )                                            1                2                                                          n            0                                              (        3        )            
If the light guiding layer is made of LiNbO3, which is one of the most commonly used electrooptical materials and has a typically achievable xcex94n of upper to 0.001, then, according to Eq. (3), its achievable maximum peak-to-peak deflection angle with the conventional waveguide optical deflector structure is only 5.33 degrees if observed in the air or only 2.45 degrees if observed inside a planar waveguide extended from the waveguide deflector. Besides, for actually achieving a large deflection angle, the width of the waveguide needs to be as small as possible, which requires the edges of the waveguide has to be very smooth; otherwise, the scattering from the edges of the waveguide tends to cause the phase distribution in the wavefront to be randomized, which deteriorates the achievable resolution for the light beam. The limitations in the achievable deflection angle and resolution make it difficult for the conventional waveguide optical deflectors to be used as the optical switch in optical routing devices.
It is noted that there are also other types of optical switches based on the electrooptical effect, such as the one having a Mach-Zehnder interferometer structure and the one that enables optical path shifting based on optical polarization changes. While they generally can switch a light beam at high speed, they usually have their own drawbacks, such as high insertion loss and high crosstalk.
On the other hand, in virtual reality retinal display in a display format similar to the one in HDTV display or Super VGA display, the required horizontal scanning resolution is as high as about 1100 and 1280, respectively. If the conventional LiNbO3 waveguide optical deflector is to be used as a scanner for the super VGA display, the width of the light beam in the waveguide has to be about 7.15 mm or wider for meeting the horizontal resolution requirement. And, with light beam width being wider than 7.15 mm, its achievable deflection angle will be smaller than the achievable maximum deflection angle of 5.33 degrees, due to the geometry limitations discussed above. The deflection angle needs to be greatly magnified so that a horizontal view angle can be as wide as at least 45 degrees for high quality display; meanwhile, the width of the light beam needs to be reduced to about 2 mm or smaller when passing the pupil of the eye. And it necessitates a sophisticated optical system to accommodate these needs simultaneously, which makes it extremely difficult, if not impossible, for the conventional waveguide optical deflector to be formed on a head-mounted display device that is comparable to an ordinary pair of eye glasses in size and weight.
Accordingly, there is a strong need for an optical deflector that can overcome the aforementioned conventional geometry limitations. And, an optical deflector that can work at high speed with high deflection angle and high resolution is not only critical to future optical devices, such as all-optical switching and high fidelity retinal image display, but also crucial to the improvement of a variety of today""s optical devices including: laser printer, bar code scanner, potable computer display, etc.
An object of the present invention is to provide an electrooptical deflector that can deflect a light beam at high speed with large deflection angle and high resolution. Another objective of the present invention is to provide a method for making the electrooptical deflector. Important aspects of the present invention include: the optical deflector has a basic structure that is readily scalable to be used, for example, as an optical switch in all-optical switching or as an optical scanner in high fidelity retinal display; It is small in size and light in weight, and, is easy to be formed in planar structure and to be integrated with other optical circuits on a common substrate; furthermore, it has a simple electrode structure and requires a low driving voltage with a simple driving scheme.
The basic structure of the optical deflector according to the present invention comprises an array of waveguide optical channels having an electrooptical layer sandwiched between a bottom electrode and one or more prism-shaped top electrode(s). When a substantially coherent light beam is split among and traveling in theses optical channels, linear phase differences are induced among the sub-light beams travelling in these individual optical channels with a voltage or a pair of voltages being applied between the top and the bottom electrodes. After these sub-light beams are emerged from the optical channels, the wavefront of the merged light beam at the far field is tilted due to the linear phase differences induced among the sub-light beams, resulting in a deflection of the light beam along a direction depending on the voltage or the pair of voltages applied to it.
The optical deflector according to the present invention overcomes the geometry limitations inherent in the conventional electrooptical deflectors by confining the light beam in the individual optical channels in the electrooptically active region. The deflection angle can be increased by increasing the ratio of the length over the total width of the optical channels, without having to worry about any over-tilting of the wavefront inside the electrooptically active region, as long as the deflection angle is within the range of xe2x88x9290 degrees to +90 degrees. Consequently, large deflection angle, high resolution and high speed can be simultaneously accomplished with the optical deflector according to the present invention.
In addition, because the electrooptically active layer is sandwiched between the top and bottom electrodes, the optical deflector according to the present invention has high deflection efficiency and requires a low driving voltage.